Dactylogyrids (Platyhelminthes, Monogenea) from the gill lamellae of doradids (Siluriformes) with description of five new species of Cosmetocleithrum and new geographical distribution for known species from the Neotropical Region, Brazil

Five new species of Cosmetocleithrum were described parasitizing the gill filaments of neotropical doradid fishes. Cosmetocleithrum undulatum n. sp., Cosmetocleithrum brachylecis n. sp. and Cosmetocleithrum ludovicense n. sp. are described from Platydoras brachylecis from a market-place of São Luís, State of Maranhão, Brazil. Cosmetocleithrum sacciforme n. sp. and Cosmetocleithrum basicomplexum n. sp. are described from Oxydoras niger from Juruá River, State of Acre, Brazil. Cosmetocleithrum undulatum and Cosmetocleithrum brachylecis resemble Cosmetocleithrum falsunilatum Feronato, Razzolini, Morey & Boeger, 2022 mainly by the unique male copulatory organ (MCO) morphology but differ from these and all congeneric species mainly by the morphology of the MCO, accessory piece and hooks pairs. Cosmetocleithrum ludovicense is closer to Cosmetocleithrum confusus Kritsky, Thatcher & Boeger, 1986 and to Cosmetocleithrum akuanduba Soares, Santos Neto & Domingues, 2018 but differs from those mainly by the morphology of the accessory piece. Cosmetocleithrum sacciforme differs from all congeneric species mainly by the morphology of the accessory piece formed by a single plate of saccular appearance. Cosmetocleithrum basicomplexum also shares morphological characters with Cosmetocleithrum gigas Morey, Cachique & Babilonia, 2019 considering the size of the body and shape of the anchors, but differs mainly in the morphology of the bars and hooks. Besides the new species, new data are presented for Cosmetocleithrum leandroi Soares, Neto & Domingues, 2018, C. akuanduba and C. confusus regarding morphological characteristics and biogeography.


Introduction
The neotropics, spanning from central Mexico to the southern limit of South America, has the most diverse group of fishes on the planet [14].This region harbors the greatest diversity of freshwater fish with approximately 6,000 known species and estimates of 9,000 species [18].Of the total known species, the Characiformes, Siluriformes, and Gymnotiformes account for approximately 77% of the known species [2].Thus, South America harbors the most diverse fauna of continental freshwater fish in the world with approximately 5,750 known species [3].Siluriformes, collectively known as catfish, stand out as the largest and most diverse order of freshwater fish, and constitute one of the most important components of the Neotropical fauna, with more than 3,800 described species [6].Among all the families of this order, Doradidae stand out as one of the most diverse and representative families among the Neotropical Siluriformes, with more than 90 valid species [6].Doradidae are a monophyletic group endemic to freshwaters of South America on both sides of the Andes Mountains [19].
Siluriformes host a remarkably rich and diverse fauna of gill monogeneans, and these host-parasite systems are an attractive model for phylogenetic studies in the Neotropics [4,12].The dactylogyrid Cosmetocleithrum Kritsky, Thatcher & Boeger, 1986 is one of the most species-rich groups of monogenoids reported from siluriform fishes and was proposed to accommodate species of Dactylogyridae that parasitize Oxydoras niger (Valenciennes) and Pterodoras granulosus (Valenciennes) in the Amazon River basin [9].This genus shows high specificity to catfishes within Doradidae and Auchenipteridae [11].Currently, 23 species of Cosmetocleithrum are recognized as parasites of the gills of neotropical Siluriformes, among which 15 species recorded in doradids hosts, eight in Auchenipteridae, one parasitizing Pimelodidae, and one in Loricariidae.All species of Cosmetocleithrum have been described from hosts of members of a single family [1, 4,5,15,22,23,24], except for Cosmetocleithrum bulbocirrus Kritsky, Thatcher & Boeger, 1986, reported from species of three different siluriform families and from Hoplias malabaricus (Bloch), a characiform fish [7].However, considering that only one specimen was found in the latter host, this record needs to be confirmed.
During research on monogeneans of siluriform fishes in northern and northeastern Brazil, five new species of Cosmetocleithrum were found and are described herein.Moreover, new data are presented to Cosmetocleithrum leandroi Soares, Neto & Domingues, 2018, Cosmetocleithrum akuanduba Soares, Santos Neto & Domingues, 2018 and Cosmetocleithrum confusus Kritsky, Thatcher & Boeger, 1986 regarding morphological characteristics and biogeographical data, thus expanding the knowledge of these species.

Material and methods
The studies were carried out between 2019 and 2023 on different doradid species collected from two distinct localities in Brazil.Specimens of Oxydoras niger were captured with gill nets and hook and line from Juruá River, Acre, Brazil (7°40 0 34.1 00 S, 72°39 0 39.5 00 W) and those from Platydoras brachylecis and Hassar affinis were obtained from street markets located on the island of São Luís, State of Maranhão (2°34 0 18.0 00 S 44°11 0 49.9 00 W).
The gills of each specimen were removed and placed in vials containing hot water (c.65 °C) in order to relax the parasites, and they were then shaken to detach the parasites from the gill filaments.Subsequently, absolute ethanol was added to reach a concentration of 70%.The vials were then sent to "Laboratório de Helmintos Parasitos de Peixes, Instituto Oswaldo Cruz, FIOCRUZ", where the gills were analyzed, and the parasites identified.Monogeneans were picked using a stereoscopic microscope for subsequent morphological studies.Some specimens were mounted in Hoyer's medium for study of the sclerotized parts; others were stained with Gomori's trichrome for study of the internal organs of the parasite [8].Measurements are presented in micrometers; range values are followed by mean and number of structures measured in parentheses.Dimensions of organs and other structures represent the greatest distance; lengths of curved or bent structures (anchors, bars, and accessory piece) represent the straightline distances between extreme ends [9], except for copulatory complexes that were measured using ImageJ [17].The specimens were studied, photographed, and drawn using an Olympus BX 41 microscope with phase contrast and Zeiss Axioskop 2 Plus microscope with differential interference contrast (DIC), both equipped with a camera lucida.Holotypes, paratypes, and vouchers of each species were deposited in the "Coleção Helmintológica do Instituto Oswaldo Cruz -CHIOC", from "Fundação Oswaldo Cruz -FIOCRUZ".Etymology: The specific name is derived from Latin (undulatum = wavy) and refers to the shape of the male copulatory organ.

Description
Based on 21 specimens: 14 mounted in Hoyer's medium and 7 stained in Gomori's trichrome.Body fusiform, elongated, comprising cephalic region, trunk, peduncle, and haptor.Kritsky, 1967, that are parasites of loracariids.However, Cosmetocleithrum undulatum n. sp.differs from C. falsunilatum regarding the shape of the MCO, which in C. falsunilatum is rolled up into a corkscrew shaped, while in Cosmetocleithrum undulatum n. sp. the MCO is an undulating tube with a thin and delicate layer that covers the entire tip of the MCO.Furthermore, the accessory piece of C. falsunilatum is subdivided into two regions that reach close to the base of the MCO, while in the new species, the accessory piece is straight, reaching just less than half of the length of the MCO.The hook pairs 1-4, 6, 7 of the new species are unique among the Cosmetocleithrum species, with a shaft that is strongly robust throughout its length, and with a small constricted portion ending with a small rounded portion at the end.
Parasitological indexes: Total number of hosts: 3; number of infected hosts: 3; total number of parasites: 38.

Remarks
Cosmetocleithrum brachylecis n. sp.resembles C. falsunilatum and C. undulatum n. sp.regarding the shape of the MCO, which is similar to the unique feature of Unilatus.While the MCO in C. falsunilatum has a cork-screw-like shape, the MCO in the new species is formed by a spiral tube with sclerotized walls.The new species differs from C. undulatum n. sp.regarding the lengths of the MCO and accessory piece: in the new species, the accessory piece is straight and elongated, such that it goes beyond the base of the cirrus, while in C. undulatum it is also straight, but only reaches just less than half of the length of the MCO.The new species also differs from all congeneric species in terms of the morphology of the hooks, which are distally slender, expanded in the middle third, and taper abruptly proximally.
Parasitological indexes: Total number of hosts: 3; number of infected hosts: 3; total number of parasites: 434.
Etymology: The specific name is in honor of people born on São Luís Island, state of Maranhão, Brazil.

Remarks
Cosmetocleithrum ludovicense n. sp. is closely related to Cosmetocleithrum confusus and to Cosmetocleithrum sobrinus Kritsky, Thatcher & Boeger, 1986 regarding the enlarged base of the MCO.However, it differs from these species in terms of the shape of the accessory piece, which in C. confusus is a hollow structure with sclerotized walls and truncated end, and in C. sobrinus is large, globose and apparently hollow, while in the new species, the MCO is formed by a thin hollow tube of inverted J-shaped, with sclerotized walls.The new species is also closer to Cosmetocleithrum akuanduba, in which the MCO has a tubular coiled shaft that frequently appears to have an inverted J-shaped but differs from the latter in terms of its sclerotized and bulbous base in the latter.
Parasitological indexes: Total number of hosts: 7; number of infected hosts: 2; total number of parasites: 7.
Etymology: The specific name is from Latin (saccus = sac; formis = shape of) and refers to the shape of accessory piece, which resembles a sac-like structure.

Remarks
Cosmetocleithrum sacciforme n. sp.differs from all congeneric species mainly in terms of the morphology of the accessory piece, which is formed by a single plate of saccular appearance.The new species is similar to C. bifurcum Mendoza-Franco, Mendoza-Palmero & Scholz, 2016 with regard to the presence of a dense mass of prostatic glands in the anterior trunk but differs in the morphology of the anchors and copulatory complex.
Parasitological indexes: Total number of hosts: 7; number of infected hosts: 2; total number of parasites: 30.
Etymology: The specific name is from Latin (basis = base; complexum = complex) and refers to the ornamentation that almost surrounds the base of the MCO.

Remarks
Cosmetocleithrum basicomplexum n. sp is more closely related to C. confusus, C. sacciforme and C. ludovicense regarding the enlarged base of the MCO, but differs from the first two in terms of the morphology of the accessory piece.In the new species, this is straight, with an apparently hollow structure and sclerotized walls opening from half its length, while in C. confusus it is a hollow structure with sclerotized walls and truncated termination and in C. sacciforme it is sac-shaped with apparently membranous walls.The new species closely resembles C. ludovicense n. sp.regarding the morphology of the copulatory complex (i.e.MCO and accessory piece) but differs in terms of the shape of anchors and bars.Cosmetocleithrum basicomplexum n. sp. shares morphological characteristics with C. gigas Morey, Cachique & Babilonia, 2019 regarding the body size and anchor shape but can be differentiated mainly by the morphology of bars.In addition, the hooks of C. basicomplexum n. sp. has a shaft that is strongly robust throughout its length, with a small constricted point, as was observed in the hooks of C. undulatum.Type-locality: Ilha grande, Xingu River, municipality of Altamira, Pará, Brazil.
Parasitological indexes: Total number of hosts: 1; number of infected hosts: 1; total number of parasites: 15.

Remarks
The morphology of C. confusus from the present study is in agreement with the original description of Kritsky et al.Type-locality: Bacajá River, municipality of Altamira, Pará, Brazil.
Parasitological indexes: Total number of host: 1; number of infected hosts: 1; total number of parasites: 93.

Remarks
Cosmetocleithrum leandroi was described from the gill filaments of Hassar gabiru, in the state of Pará, Brazil.According to the original description, C. leandroi is characterized by a MCO comprising a coil of about 3½ rings, a sigmoid accessory piece with a cup-shaped distal portion, a single type of hooks, and anchors with poorly differentiated roots.The specimens studied here, based on newly collected specimens from H. affinis, were in accordance with the original description, while presenting some differences in morphology and measurements, compared with the original material, deposited in CHIOC (Holotype 39053 a, paratypes 39053 b-f, vouchers 39054 a-c): body length 933 (650-1,250) long by 119 (110-175) wide, MCO with 3½ to 4½ counter-clockwise rings, ventral and dorsal anchors with pointed superficial root and deep root developed and hooks with slightly dilated shank, in the present material vs body length 712 (575-835) long by 132 (102-157) wide, MCO with 3½ counter-clockwise rings, superficial and deep roots poorly developed and hooks with non-dilated shank in the original description and type material examined.

Discussion
Cosmetocleithrum is one of the most diverse genera of dactylogyrids parasitizing neotropical catfishes [4].It is characterized by the presence of two submedial ribbon-like projections on the dorsal bar, a copulatory complex comprising a variably coiled cirrus with counter-clockwise rings and an elaborate accessory piece non-articulated to the cirrus base [10].
So far, species of Cosmetocleithrum have been found in fishes caught in Argentina (1), Brazil (18), and Peru (4).Among these, 14  Nevertheless, in the phylogenetic analysis of Cosmetocleithrum conducted by Mendoza-Palmero et al. [11], the phylogenetic position of the species analyzed corresponded partially to the morphological categories proposed by Cohen et al. [4].
Their study [11] provides suggestions for further studies regarding Cosmetocleithrum spp., including the molecular characterization of the remaining species of this genus in order to evaluate the phylogenetic positions of all the species.
Mendoza-Palmero et al. [11] stated that more than 15 species of Cosmetocleithrum have been described over recent years, thereby adding new morphological characteristics that can be included in the diagnosis of the genus.They cited C. bifurcum (a member of the "doradid group") and C. baculum (a member of the "auchenipterid group"), which have dissimilar hooks, but C. nunani (also a member of the auchenipterid group) presents two morphologically distinct hooks.Regarding the species described in the present paper, C. undulatum (a member of the doradid group) can be included in this set of species with two morphological distinct hooks, thus showing that it is not related to the host family.Considering the position of the vagina, a feature also mentioned by Mendoza-Palmero et al. [11], the species described herein are concordant with almost all species of the genus, presenting a sinistral aperture, with the exception of C. tortum.
Despite the high diversity of catfish in the Amazon region and the economic importance of some of these species, knowledge of the helminth fauna parasitizing these fish is still fragmentary and far from sufficient [12].Moreover, no studies on the helminth fauna of many of these fish have yet been conducted, as is the case of P. brachylecis, which is a thorny catfish endemic to the basins of the Mearim River, Pindaré River, Itapecuru River and Parnaíba River, in northeastern Brazil [16].The new species described in the present study and the new records of hosts for P. brachylecis demonstrate that there is a real need to expand such studies, especially with regard to endemic fish species and those that have recently been described.The finding, more than 30 years later, of two new species in O. niger, the type host of the first four species that were proposed for Cosmetocleithrum, also demonstrates that studies on these hosts are necessary and should take into consideration the ecological processes related to the host-parasite association.